The present invention relates generally to the printing of webs, and more particularly to the printing of traveling webs. Still more particularly, the invention relates to an apparatus for printing a traveling web.
It is known to print traveling webs in printing machines, usually of the screen printing type. Especially if the web is a textile web, and if a screen printing machine is used, it is customary to apply the web onto a traveling printing blanket of the machine, which serves to support the web. Usually, the web is adhesively secured (by means of adhesive which allows it later to be stripped off) to the printing blanket, or else the printing blanket is provided with needle projections which penetrate into the web (this is usually done when the web is of a heavy quality, for instance a carpet). The purpose is to assure an absolute uniformity of the movements of the web and the printing blanket; in other words: no relative displacement of printing blanket and web is to be allowed to occur.
Theoretically, this approach works well. In actual fact, however, difficulties are frequently experienced, especially if the web is of a relatively heavy character, for instance carpeting or the like. Heavier textile webs, such as carpets, cannot be so produced that they are absolutely uniform in tension over their entire width. As a general rule, the edge portions of such a web are woven more loosely than the center portions, or sometimes the reverse might be true. In any case, it is well known that if a web which has this differential characteristic is advanced in the manner outlined above through a screen printing machine, bulges will frequently form in the web when it passes beneath the printing screens, with the result that the image or pattern that is being printed onto the web will become blurred and unsightly. The reason for this is that the web will be locally squeezed as it passes under the printing screen and an unsightly print will be obtained, or a local relative displacement will occur between portions of the web that are displaced by contact with the printing screen, and the printing screen and printing blanket.